Nonwoven fabric sheets having two layers of a spunbonded nonwoven fabric and a meltblown nonwoven fabric are relatively durable and have good extraction performance in general. The nonwoven fabric sheets are thus suitable for the material of filters or bags for extraction and widely distributed. Examples of such filters and bags include drip bags sealingly filled with powder materials such as powder coffee, tea bags sealingly filled with tea leaves such as black tea and green tea, soup stock packs sealingly filled with chips of materials such as kelp and dried bonito, bathing bags sealingly filled with bath additive fragments, and drug bags sealingly filled with powdered decoction.
As forming machines and forming filling machines for manufacturing these extraction filters have been improved for high-speed operation with the progress of technology year by year, the needs of short-time sealing during the manufacturing of extraction filters by, for example, cutting and sealing nonwoven fabric sheets are increasing. Therefore, the development of nonwoven fabric sheets for extraction filters with adequate sealing strength in a short time is demanded.
For the nonwoven fabric sheets having two layers of a spunbonded nonwoven fabric and a meltblown nonwoven fabric, in order to obtain adequate sealing strength in a short time of sealing these two materials with the meltblown nonwoven fabric layer (sealing layer) placed inside, the meltblown nonwoven fabric layer requires that the fibers forming the meltblown nonwoven fabric is readily softened and fluidized by heating, and that the fluidized resin readily infiltrates into the space among fibers in the spunbonded nonwoven fabric and integrates with the same. In order to satisfy these conditions, the resin forming the meltblown nonwoven fabric must have a low softening point, and have adequate fluidity upon the melting. In contrast, the spunbonded nonwoven fabric needs to have thermal resistance capable of maintaining fiber form against heat and pressure upon the sealing. In order to achieve such characteristics, the resin forming the spunbonded nonwoven fabric must have an adequately higher softening point than that of the resin forming the meltblown nonwoven fabric, and therefore, the spunbonded nonwoven fabric must maintain the fabric form without deformation upon being exposed to such a high temperature that the meltblown nonwoven fabric will melt.
As used herein, the term “sealing strength” refers to the breaking strength of a sealing part formed by sealing two nonwoven fabrics in a welding manner.
There exist mainly three types of the nonwoven fabric sheets having two layers of a spunbonded nonwoven fabric and a meltblown nonwoven fabric. The first type is a nonwoven fabric sheet manufactured by independently preparing the spunbonded nonwoven fabric and the meltblown nonwoven fabric, and then overlaying one on the other and adhering to each other by partial thermocompression bonding or other processing (type A). The second type is a nonwoven fabric sheet manufactured by performing partial thermocompression bonding onto the spunbonded nonwoven fabric, and blowing a heat-melted resin in fibrous form onto a surface of the spunbonded nonwoven fabric to laminate the meltblown nonwoven fabric thereon (type B). The third type is a nonwoven fabric sheet manufactured by blowing a heat-melted resin in fibrous form onto a surface of a web-shaped spunbonded nonwoven fabric without partial thermocompression bonding to laminate the meltblown nonwoven fabric thereon (type C).
Because the nonwoven fabric sheet of type A is integrated together after one nonwoven fabric is overlaid on the other, they only adhere to each other partially by partial thermocompression bonding or other processing. The nonwoven fabric sheet of type A therefore has a low adhesive strength on an interface between the spunbonded nonwoven fabric and the meltblown nonwoven fabric, so that it may fail to obtain adequate sealing strength.
The nonwoven fabric sheet of type C employs the web-shaped spunbonded nonwoven fabric formed from fibers that do not adhere to each other. Thus, the nonwoven fabric sheet of type C has low tensile strength and easily break or deform, which makes it difficult to obtain high sealing strength.
In order to increase the tensile strength, it is contemplated that the nonwoven fabric sheet of type C is subjected to partial thermocompression bonding to cause the fibers in the spunbonded nonwoven fabric to adhere to each other after the lamination of the meltblown nonwoven fabric onto the surface of the spunbonded nonwoven fabric. In such a method, the laminated meltblown nonwoven fabric has a lower softening point than that of the spunbonded nonwoven fabric, which makes it difficult to control the temperature such that the spunbonded nonwoven fabric is softened to such an extend as to integrate together without excessive softening of the meltblown nonwoven fabric. Consequently, the partial thermocompression bonding for the nonwoven fabric sheet of type C is incomplete, and the nonwoven fabric sheet of type C will often fail to obtain adequate tensile strength.
In contrast, the nonwoven fabric sheet of type B is manufactured by previously performing partial thermocompression bonding onto the spunbonded nonwoven fabric to cause the fibers therein to partially adhere to each other and integrate together, and then blowing a heat-melted resin onto the surface of the spunbonded nonwoven fabric to laminate the meltblown nonwoven fabric thereon. The nonwoven fabric sheet of type B can therefore have high adhesive strength on the interface between the spunbonded nonwoven fabric and the meltblown nonwoven fabric, resulting in high sealing strength and high tensile strength.
The inventors of the present invention have selected the above-described nonwoven fabric sheet of type B as a target for development, and also have strived to develop a nonwoven fabric sheet having an adequate sealing strength in fast forming machines.    Patent Literature 1 discloses a drip-type package for filling beverage materials having a water-permeable bag body configured of an outer layer and an inner layer, the outer layer being a spunbonded nonwoven fabric containing 50% or more of polyolefin and the inner layer being a meltblown nonwoven fabric made of a polyolefin.
The package, however, employs the above-described nonwoven fabric sheet of type A manufactured by independently preparing the spunbonded nonwoven fabric and the meltblown nonwoven fabric, and then overlaying one on the other and adhering to each other. The package therefore has low adhesive strength on the interface between both nonwoven layers, so that it may fail to obtain adequate sealing strength.    Patent Literature 2 discloses a nonwoven fabric for filters in which a meltblown nonwoven fabric made of polybutylene terephthalate or polytrimethylene terephthalate fibers each having a fiber diameter of 1 to 8 μm is integrally laminated onto a spunbonded nonwoven fabric made of polyester-based fibers having a fiber diameter of 10 to 30 μm.
This nonwoven fabric for filters, however, is intended to improve dust collection performance, and the integration of the meltblown nonwoven fabric and the spunbonded nonwoven fabric is preferably achieved with partial thermocompression bonding by using a pair of hot emboss rolls and the like. This nonwoven fabric also falls within the category of the above-described type A. The nonwoven fabric also has a small difference in the melting point between the resin forming the meltblown nonwoven fabric and the resin forming the spunbonded nonwoven fabric, and thus may fail to obtain adequate sealing strength.    Patent Literature 3 discloses a filter for food having a laminate nonwoven fabric that is integrated by laminating a low-crystallinity superfine fiber nonwoven fabric layer formed by meltblowing on a long fiber nonwoven fabric layer formed by spunbonding, and then performing inline partial thermocompression bonding to integrate together.
This filter for food employs the above-described nonwoven fabric sheet of type C, and is manufactured by entangling superfine fibers with a web-shaped long-fiber nonwoven fabric in which long fibers do not adhere to each other, or are fixed, and then performing partial thermocompression bonding to adhere the long fibers to each other. This makes it difficult to control the temperature such that the super fine fibers are softened to such an extent as to adhere the long fibers to each other without excessive softening. Consequently, the partial thermocompression bonding for the filter for food is incomplete, and the filter may fail to obtain adequate tensile strength and therefore fail to have high sealing strength.